The field of smart materials and intelligent structures has been gradually developing over the past few decades, increasingly enabled by technological advances in the areas of sensors, engineering materials, and actuators. The basis of many actuator and sensor technologies has increasingly been found in emerging “variable stiffness materials.” Variable stiffness materials, as a category, are materials that change their shape in response to an external control stimulus, typically a field, such as a thermal, magnetic, or electric field, but also radiation (light) or a changing chemical environment. Materials in this broad category include several classes, often delineated by the stimulus arid material type: shape memory alloys (SMAs), shape memory polymers (SMPs), piezoelectric ceramics, magnetostrictives, and electroactive polymers. Within each of these classes, there are many materials; e.g., within electroactive polymers alone there are a wide variety of low- and high-voltage-activated materials with widely-varying properties, such as ionic-polymer metal composites, conductive polymers, gels, and others.
Additionally, deployable and/or deformable structures have been obtained using variable stiffness materials incorporated with linear elastic materials. Moreover, variable stiffness segmented composite materials have been developed.
In variable stiffness segmented composite materials, there are two components. These two components are the variable stiffness matrix component and the reinforcement component. In operation, a variable stiffness segmented composite material can be used in two operation modes. In the first operation mode, the variable stiffness matrix component effectively connects the reinforcement component elements to provide large stiffness. In the second operation mode, the variable stiffness matrix component is dramatically softened (or otherwise changed in state) to effectively disconnect the reinforcement component elements. In this second operation mode, large deformation can be achieved.
However, deformable structures that undergo operation or performance enhancing shape changes and have zero-power-hold functionality have limited utility in thin, planar form factors (such as morphing skins) if undesirable deformations are not prevented, accommodated or recovered. An example of such deformable structures with the above functionality is a covering made of variable stiffness materials incorporating shape memory polymers as shown in FIGS. 1a, 1b, and 2. For variable stiffness segmented composite materials (hereafter also referred to as “variable stiffness materials,” “VSMs,” or “variable stiffness composites”), a typical composite microstructure is a laminate composed of alternating layers of a stiff, structural material such as (but not limited to) steel or aluminum, and a shape memory polymer or other polymer such as epoxy, vinyl ester or polyester variants. The shape memory polymer can be of the thermoplastic or thermoset type. The thermoset shape memory polymer precursor is a liquid mixture of resin and curing agent, and can be applied via wet lay-up or infusion methods. Other polymers and elastomers or may be used as a matrix material.
During operation or performance enhancing shape changes, the variable stiffness material covering becomes soft, and compressive stresses or out-of-plane operational loads may induce undesired out-of-plane deformations as shown in FIGS. 3a and 3b. As such, a continuing need exists to mitigate these undesirable deformations.